116
UNIT 2
Basic Tissues
Figure 7-12A
Dura Mater, Arachnoid, and Pia Mater
Figure 7-12B
Spinal Meninges
Figure 7-12C
Clinical Correlation: Meningitis
Figure 7-13A
Types of Glial Cells
Figure 7-13B
Astrocytes
Figure 7-13C
Clinical Correlation: Glioblastoma
Autonomic Nervous System
Figure 7-14
Overview of the Autonomic Nervous System
Figure 7-15A
Sympathetic Ganglion
Figure 7-15B
Myenteric Plexus (Auerbach Plexus)
Figure 7-16
Submucosal Plexus (of Meissner)
Table 7-1
Comparison of Posterior Root and Autonomic Ganglia
Synopsis 7-1
Pathological and Clinical Terms for the Nervous System
Introduction and Key Concepts
for the Nervous System
It is difF cult to consider the tissue of the nervous system
separately from the nervous system itself. In most organ sys-
tems, the purpose of the tissue is to F
lter, secrete, or transfer
gases or digest and absorb nutrients. The histological structure
of one small region of the liver or kidney or small intestine is
very much like the structure of any other region of that organ,
and the function of one portion of the organ is very much like
the function of any other portion. By contrast, the purpose of
the nervous system is to carry sensory information from the
sensory organs to the brain; to process that sensory informa-
tion in the brain to produce perceptions, memories, decisions,
and plans; and to carry motor information from the brain to
the skeletal muscles in order to exert an infl
uence on the indi-
vidual’s surroundings. In truth, all we know of the world that
surrounds us is carried as electrical impulses over our sensory
nerves; the only way we have of interacting with that world is
via electrical impulses carried by motor nerves from our brains
to our muscles.
Neurons and Synapses
The building blocks of the nervous system are cells called
neurons
. These cells have a long, thin process, the
axon
, in
which the cell membrane incorporates specialized protein
ion
channels
that enable the axon to conduct an electrochemical
signal (
action potential
) from the cell body to the
axon termi-
nals
. Axon terminals of one neuron make synaptic contacts
with other neurons, generally on processes called
dendrites
or
on the cell body itself. When the action potential reaches the
axon terminals, a
neurotransmitter
is released from
synaptic
vesicles
into the
terminals. The neurotransmitter molecules act
on
receptor
molecules
that are part of ion channels in the den-
drites and soma of the next neuron in a chain. The constant
interplay of excitatory and inhibitory infl uences at the many
billions of synapses in the nervous system forms the basis of our
ability to be aware of our surroundings and to initiate actions
to infl
uence our surroundings (±igs. 7-1 to 7-3).
Overview of the Peripheral and
Central Nervous Systems
By deF nition, the brain and spinal cord are classiF
ed as the
cen-
tral nervous system (CNS),
and the nerves and ganglia outside
these structures are classiF
ed as the
peripheral nervous system
(PNS)
. Collections of axons that carry action potentials from
one place to another are called
nerves
in the PNS and
tracts
within the CNS. Clusters of neuron cell bodies are called
gan-
glia
in the PNS and
nuclei
or
cortices
in the CNS (±ig. 7-4).
Peripheral Nervous System
Nerves in the
PNS
carry sensory information from receptors
located in the skin, muscles, and other organs and carry motor
commands from the CNS to muscles and glands. Nerves consist
of clusters of axons surrounded by protective connective tis-
sues (±ig. 7-5A). Nerve axons range in diameter from about
0.5 to 22
μ
m, with the conduction velocity being higher for
larger axons. In addition, larger axons generally have a dense,
lipid-rich coating,
myelin
, which further increases conduction
velocity (±ig. 7-6). The cell bodies associated with the sensory
neurons are clustered in a swelling of the posterior spinal root,
the
posterior (dorsal) root ganglion
.
Central Nervous System
The spinal cord consists of large bundles of myelinated and
unmyelinated axons arranged into
ascending
(
sensory
) and
descending
(
motor
)
tracts
(±ig. 7-9A). The
ascending tracts
carry information from peripheral receptors to nuclei in the
brainstem and thalamus and from there to the cerebral cortex.
The
descending tracts
carry motor information from the cere-
bral cortex and motor centers in the brainstem to interneurons
(relay neurons) in the motor pathways and directly to spinal
motor neurons. These motor neurons innervate muscles directly
to produce movement. The tracts are clustered around a central
region, the
spinal gray matter
, which contains large numbers
of
sensory
and
motor interneurons
,
spinal motor neurons
, and
preganglionic autonomic visceromotor neurons
.
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